Counterbalancing systems for sectional overhead doors have commonly employed torsion spring arrangements. The use of torsion springs in such sectional overhead doors is, in significant part, because the linear tension characteristics of a torsion spring can be closely matched to the substantially linear effective door weight as a sectional door moves from the open, horizontal position where the door is largely track supported to the closed, vertical position or vice versa. In this manner, the sum of the forces acting on such a sectional garage door may be maintained relatively small except for momentum forces generated by movement of the door by the application of manual or mechanical forces. In this respect, sectional overhead doors have been provided with lift cables or similar flexible elements attached to the bottom of the door and to cable storage drums at the ends of a drive tube, which rotate when the drive tube is actuated.
In many cases, these cable storage drums have surface grooves that guide the lift cables on and off of the cable storage drum to prevent the coils or cable wraps from rubbing against each other and chafing if positioned in side-by-side engaging relationship or if coiled on top of each other. Lift cables sized to meet operational requirements for sectional overhead door applications are commonly constructed of multiple strand steel filaments that have a pronounced resistance to bending when stored on the circumference of the cable drums and, thus, require tension to remain systematically coiled or wrapped about the cable drums in the grooves therein.
A problem arises if tension is removed from one or both of the lift cables of a sectional overhead door in that the lift cables tend to unwrap or separate from the cable drums; thereafter, when tension is restored, the lift cables may not relocate in the appropriate grooves or the appropriate relation to adjacent cable wraps. In some instances, a cable wrap will locate on a groove further inboard of the door from its original position so that as the door moves to the fully opened position, the cable drum runs out of grooves for cable wraps, such that the lift cable coils about parts of the drum that are not designed for cable storage. In this instance, if the lift cables dislodge from the cable storage drum and engage the smaller radius of the counterbalance system drive tube, the leverage effected by the springs is reduced such that the door will be extremely difficult or impossible to move. This is because the linear force between the door and the counterbalance springs relies on the leverage against the counterbalance spring being applied by the weight of the door operating through the radius of the cable storage drum rather than at a reduced radius portion of the cable drum or the drive tube for the counterbalance system.
In other instances, the removal of tension from the lift cables can result in cable wraps or coils being displaced to overlie existing cable wraps stored on the cable drum, which may cause the length of cable between the cable drums at opposite ends of a door to assume a different effective operating length. In such case, the door may be shifted angularly in the door opening, with the bottom edge of the door no longer paralleling the ground and the ends of the door sections moving out of a perpendicular orientation to the ground. If thus oriented, continued movement of the door can readily result in the door binding or jamming in the track system and, thus, being rendered inoperative.
In the instance of either of these operating anomalies occasioned by loss of tension in the lift cables, it is probable that the resultant tangling of the lift cables and/or jamming of the doors will prevent the door from further automatic or manual operation, leave the door in a partially open condition, and require qualified service personnel to repair damaged components and realign and assemble the door and counterbalance system components before the door is restored to normal operating condition.
There are a number of possible operating circumstances wherein tension in the lift cables of a counterbalance system for a sectional overhead door becomes reduced to such an extent that the lift cables may become mispositioned on or relative to the cable storage drums, thereby producing the problems discussed above. One example is when a door is rapidly raised from the closed to the open position at a velocity that is faster than the cable storage drums can rotationally react, such that slack is created in the lift cables. Another example is in the utilization of a motorized unit that turns the counterbalance system shaft to open and close a sectional overhead door, such as installations that employ what are termed in the trade as "jack-shaft operators". A jack-shaft may create cable slack when the operator turns the cable storage drum without the door moving, or the door is manually moved without actuating the cable storage drums.
The primary approach to preventing cable mispositioning has involved utilization of grooves in the circumference of the cable storage drums, which are otherwise present for positioning and spacing cable as it is taken up during the raising of a garage door. In some instances, exaggerated or deep grooves have been employed in the cable storage drums in an effort to maintain the lift cables appropriately positioned during a loss of tension on the lift cables. While the use of grooves so configured may be helpful in preventing lift cable mispositioning in minor losses of tension, this approach does not solve the commonly encountered problem of appreciable slack being created in the lift cables.
Another approach to avoid lift cable mispositioning in the event of the creation of cable slack is the use of cable slack take-up devices that compensate for cable slack when it occurs. A device of this type may employ a spring-loaded arm that displaces the cable in a controlled direction to take up any cable slack that might occur, with the controlled direction permitting proper repositioning of the lift cable on the cable storage drum once the slack is operationally eliminated. Normally, however, these designs will take up only minimal amounts of cable slack, and the cable take-up devices, if sensitive enough to be effective, impart a vague or detached component that derogates the desired positive drive positioning of the door during raising and lowering operations. These cable slack take-up devices also tend to require frequent adjustment as a function of component wear of the various components of the cable take-up device.
Another approach to eliminating the problem of cable slack in lift cables contemplates the use of an additional cable or cables connected to the top, as well as the conventional cables connected to the bottom, of a sectional overhead door to create what is sometimes referred to as a closed loop system, wherein the door is pulled open by one lift cable or cables and pulled closed by another cable or cables, with the cable storage drums for all of the cables being attached to the same counterbalance system drive shaft. Attempts to employ this closed loop system design results in the necessity for additional pulleys and hardware at substantial additional cost. In addition, the speed of the two points of attachment to the door are not uniform relative to the drive shaft, at least in areas where the top of the door is traversing the radius from the vertical to the horizontal storage position, while the bottom of the door is moving purely vertically. Such a speed differential requires compensation, such as a spring, which nonetheless may produce notable resistance to door motion. In some instances, the cables of a close loop system may contact the face of the door during a portion of the door travel, which can produce an unsightly mark on the face of the door that is visually apparent on the outside of the door when the door is in the closed position. Thus, no solution to cable slack in sectional overhead door systems having motor-driven counterbalance systems has achieved wide acceptance in the industry and, therefore, motor-driven counterbalance systems for sectional overhead doors have enjoyed only limited usage in the industry.